A filter press generally comprises a vertical fixed resisting end plate and a head resisting plate, connected by external parallel beams. Between the end and head plates, a mobile resisting plate is moveably supported on the beams and is moved by a powerful double effect jack, fixed on the head resisting plate. A plurality of filtration members are supported on the beams between the end and mobile plates and are slidably mounted on the beams. These filtration members are pressed tight against each other or are spaced apart from each other by the thrust or retraction of the jack acting on the mobile plate. The filtration members can be filtration plates and frames or recessed plates.
According to conventional techniques, the filtration plate has, on its periphery, a certain width which forms a flat sealing surface for sealing the filter press when closed. Between the sealing surfaces, the plate is grooved or pipped on both sides. Both faces of the plate are covered by a filter cloth.
The frame joining the plate is of the same size as the sealing surfaces. When the filter press is sealed closed, slurry is injected under pressure into the frame. The liquid or filtrate traverses the filter cloths, flows into the grooves in the plates, and is evacuated through apertures in the bottoms of the plates. The solid material or cake is retained by the filter cloths.
When the frame is filled by the filter cake, the cake can be desalted by washing and dewatered by an important compressed air blowing operation. Subsequently, the filter press is opened by retraction of the jack and the mobile resisting plate.
To remove the filter cakes, the plates and frames must be separated. The separation can be accomplished by various means, such as chains, hooks, distancers, a mobile pendulum, or other means. After the cakes are removed, the filter presses are reclosed to start another filtration cycle.
The filtration members can also be recessed or chambered plates which can operate without frames. The two faces of these thicker plates are hollow between the sealing surfaces at the periphery to define a chamber inside of the sealing surfaces. The bottom of this hollow portion is grooved or pipped. These recessed plates are also covered by a filter cloth.
When two plates are tightly closed against each other, the cake is formed in the two adjacent chambers forming and operating as a frame. However, it is more difficult to inject slurry into these chambers than in a frame system. These recessed plates perform the same function as the plates and frames discussed above.
To dewater the cakes and to avoid the expensive air blowing operation, membranes have been provided in conventional filter presses to squeeze the cakes. However, the plates used in such conventional systems are full plates formed of completely rigid material, such as steel, cast iron, aluminum, plastics or the like. Such plates with membranes are heavy and are difficult to move along the beams of the filter press. In order to move such plates, very strong jacks or other moving means must be employed.
If the slurry is fed into one of the two frames next to a plate and the feed is blocked to the second frame, a high differential pressure is exerted on the grooved part of the plate. Such high differential pressure breaks or deforms the plate rendering the plate unusable. This also requires the expensive replacement of the plate. The same frequent accident also occurs with chambers of joined recessed plates.
It is also known to feed the slurry through one hole made in the grooved part of the plate. In this case, the filter cloths must be safely tightened around this hole by fixing pieces. This fixing of the filter cloths takes extensive time during filter cloth replacement. If a membrane is installed under the filter cloth, it also must be tightened by the same pieces as shown in U.S. Pat. No. 3,503,326. However, compressed air supplied to the back of the membranes to inflate them for squeezing the cakes will tend to tear the membranes out of the fixing pieces. Consequently, compression of the filter cake can only be achieved at low pressure. Additionally, the membranes cannot be uniformly inflated due to this means of fixing the filter cloth.
As disclosed in U.S. Pat. No. 3,503,326 in FIG. 7 thereof, it is also known that only one membrane can be used with a plate, reducing the compression efficiency. Compression efficiency will be significantly enhanced with the use of two membranes to squeeze one cake.
The membranes such as those in the above-mentioned U.S. patent are smooth. Other conventional membranes are grooved or pipped on one side. It is impossible to filter on the side of a smooth membrane since inadequate space is provided for the flow of the filtrate or liquid. Consequently, a cake is filtered by one filter cloth only, adversely affecting the productivity of the filter press.
It is further known that membranes are not fitted to the plates, but are made part of separate membrane supports or bearers. The membranes are formed by a bag inside the bearer; see for example, U.S. Pat. No. 3,503,326.